Millimeter wave generating reflex klystron



Feb. 11, 1958 H. LEBOUTET MILLIMETER WAVE GENERATING REFLEX KLYSTRON 2Sheets-Sheet 1 Filed May 13, 1955 FIG. 1

Feb. 11, 1958 H. LEBOUTET 2,823,334

MILLIMETER WAVE GENERATING REFLEX KLYSTRQN Filed'May 13, 1955 2Sheets-Sheet 2 FIG- 2 United States Patent MILLIMETER WAVE GENERATINGREFLEX KLYSTRON Hubert Leboutet, Paris, France, assignor to CompagnieGenerale de Telegraphic Sans Fil, a corporation of France ApplicationMay 13, 1955, Serial No. 508,090. Claims priority, application FranceMay 18,1954

7 Claims. (Cl. 3155.22)

The invention relates to millimeter wave generators using reflexklystrons.

It has already been proposed to obtain a millimeter wave generator tubeof the reflex klystron type by so selecting the size of its cavityresonator that its fundamental resonant frequency corresponds to thedesired millimeter wave. Practically insurmountable difliculties havebeen encountered, however, owing to the small geometric dimensionsrequired for the various constituent parts of the tube. For example, acavity resonator of the rhumbatron type dimensioned for resonance at afundamental frequency of 3 mm. would have all its dimensions equal to,at the most, 1.5 mm. These small dimensions necessitate very thinelectron beams and thus very weak electronic currents.

It has also been proposed to obtain millimeter waves by utilizing acentimeter wave klystron generator followed by a series of crystalfrequency multipliers. This system, however, only affords very low powerwhich is always less than the power furnishedat the fundamentalfrequency by the klystron. Furthermore, the gen-- erating device isextremely complicated.

The object of the present invention is to provide a reflex klystrondevice oscillator dimensioned for a fundamental frequency of oscillationpertaining to the range of centimeter waves and designed in such mannerthat at least one of its harmonics pertaining to the range of millimeterwaves and, in consequence, of the orderof at least 10, is favored, onlythe energy corresponding to Fig. 1 is an axial sectional view of theklystron embodying the invention. 7

Fig. 2 is a cross-sectional view, taken along line II-II of'Fig. 1, ofthe elements of the klystron situated above this section.

The reflex klystron shown in Fig. 1 has a structure of revolution. Itcomprises at its lower part a cylindrical metal envelope 1 in the formof a body of revolution. closed at its lower part by a glass envelope 2and at its upper part by a hollow truncated cone 3 of metal which issupported by the envelope 1 through the medium of a metal ring 40 and isprovided at its lower and upper ends with a first opening 43 and asecond opening 13. A cavity resonator 41 in the shape of a rhumbatron isbounded by the conical element 3, a flat metal element.4, and deformablemetal walls 5. A third opening formed in the element 4 is in axialalignment with the two firstmentioned openings 43 and 13.

Coaxial with the tube and disposed inside the cylindrical envelope 1, isan electron gun comprising a cathode 6, heated by a filament 7, and ananode 8 The supply leads for the electrodes 6, 7 and 8 extend throughthe glass envelope 2 and are respectively supplied by a source of directcurrent voltage 9, in the case of the fila- "ice ment 7, and a source ofdirect current voltage 10 adjustable by means of a potentiometer 11, inthe case of the anode 8.

The upper part of the reflex klystron comprises a cylindrical metalenvelope 17 fixed to the metal element 4 and closed at its upper part bya glass wall 18.

The interior of the three portions 1, 41 and 17 is evacuated.

A metal rod 42 coaxial with the tube and disposed inside the cylinder 17supports a reflector 19. whose distance to the upper base of the cone 3is designated by h. A source 20 supplying a continuous voltageadjustable by means of a potentiometer 21 applies to the reflector 19the appropriate direct current negative voltage.

As the walls 5 of the cavity resonator 41 are deformable, the distance dbetween the two openings 13 and 15 inside the cavity resonator may bevaried. The adjustment of the distance d is ensured by a mechanism ofknown type comprising a housing 22 fixed to the metal envelope 1enclosing the gun 6 by means of a plate 23 rigidly connected to thisenvelope. The housing 22 encloses a ring 24 provided with external teethand an internal screw thread; this ring 4 is rotatable between balls 25.The internal screw thread meshes with a ring 26 rigid with the envelope17. The outer periphery of the ring 24 meshes with a worm gear 27. Onrotating the latter, the ring 26 is angularly displaced about its axisand the elements 17 and 4 unitary with this ring move with the latter,the elements 1 and 3 remaining stationary. The wall 5 is deformed andthe distance d is varied.

The above-described elements are similar to those to be found in reflexklystrons of known type. The klystron embodying the invention is new inso far as concerns the special structural features now to be described.

The opening 13 is provided with two groups of four radial fins 14 and14, the two groups having different.

radial dimensions 7\ /2 and 1 /2 respectively, of the order of onemillimeter. Provided below this opening in the inner wall of the hollowmetal cone 3, is a wave trap 33. The opening 15, which has preferably adiameter slightly greater than the opening 13, is also provided with twogroups of four fins 16 and 16', the dimensions being substantially A /2for the first, and A /2 for the second group, as shown in Fig. 2.

The cavity resonator 41 is provided with five microwave outputs as shownin Fig. 2: an output 30 so dimensioned as to transmit energycorresponding to its fundamental frequency range of oscillation centeredon a wavelength A in free space and two pairs of outputs 28 and 29respectively dimensioned for transmitting two ranges of wavelengths bothinferior to x /lt) and respectively centered on x and the wavelengths xand x being equal to the above-defined dimensions x and The output 30 isa coaxial antenna terminating in the cavity resonator 41, in a loop 39,the plane of which is perpendicular to the plane, of Figure 2. It isloaded to an adjustable impedance 34 through a matching device 35 whichis so adjusted that the load impedance seen at the output of the antennais a practically pure reactance.

The outputs 28 and 29'are constituted respectively by two pairs of waveguides, the two guides of each pair having the same section and theguides of one pair 28,

for example, having a section greater than the guides of the other pair29. Each guide is closed by a very thin glass window 36. In the specificexample shown in Fig. 2, the useful energy in the range A is obtainedfrom the guide I 29 having an axis 0A, the other guides terminating in ashort-circuit piston 37. The tube could be adjusted, as

3 Two coils 38 are disposed around the tube for producing a weak axialmagnetic field 4 (Fig. 1).

The main dimensions of the klystron shown in Fig. l are governed by thefollowing parameters:

a designates the outside radius of the upper base of the cone 3.

b designates the radius of the cavity resonator.

d is the above-mentioned distance between the openings 13 and 15 insidethe cavity resonator.

h is the above-mentioned distance between the reflector and said upperbase.

I is the mean distance between the plate 4 and the flexible deformablewall 5.

The above-described klystron operates in the following manner:

The electron beam produced by the cathode 6 follows, under the action ofthe accelerating voltage of the anode 8, a path passing through insuccession: space 32 inside the envelope 1 between the anode 8 and theopening 43, and the space 31 inside the hollow metal cone 3. The beamthereafter enters the cavity resonator 41 through the opening 13. Itpasses through this cavity resonator in which it is velocity modulatedin a direction parallel to the axis of the tube and emerges from theopening 15. It approaches the reflector 19 and the latter, brought to anegative potential, repels the beam which follows, in the oppositedirection, a path parallel to that which it had just followed, enteringthe cavity resonator 41 through the opening 15 and emerging therefromthrough the opening 13. As is well-known, a density modulation of thebeam occurs in the space between the plate 4 and the reflector 19. Ahigh frequency fiel-d having components of various resonant frequenciesis produced and is maintained in the cavity resonator 41 by the bunchedbeam. The above-described operation is similar to that of a conventionalreflex klystron. The characteristics of the klystron embodying theinvention as concerns dimension, adjustment and operation will now bedescribed.

The cavity resonator is so constructed and dimensioned to fulfil thefollowing conditions:

(1) The cavity is capable to resonate, whatever the distance d may be,within its limits of variation, on certain nearly constant frequencies,corresponding to higher modes, in the two ranges around A and A (2) Thecavity is tunable, the fundamental frequency depending upon theadjustment of distance d, comprised for example in the centimeter range.

(3) For a few values of the distance d, certain of said fixed higherfrequencies are true harmonics of the fundamental frequenciescorresponding respectively to said values.

(4) The cavity is capable of providing an important part of the energyat said fixed frequencies, in its reentrant portion, so as to allowinteraction between the beam and said energy.

It has been found that when these results are obtained, the followingequation (which is an approximate equation but nevertheless valid over alarge range) established by the applicant:

p b em 6 1 2 2 k m 2 i, 2.62 (m-1) m-1)'V 4(1) 1 m-1 is satisfied withintegers n20, and k21.

In this equation: A (wavelength in free space of the fundamentalfrequency) being given, the sizes of the cavity resonator are calculatedin a known manner, for example from the annex B of the work KlystronTubes by A. E. Harrison, 1947:

designates the order of harmonics it is desired to generate. e Is equalto and m is equal to The values of the various parameters in thisformula, which depend on the dimensions of the klystron, have been soselected in the construction of the latter that while adjustingthevalues of d, by means of screw 27, it is possible to obtain a fewnumber of groups of satisfactory values for n and k. Thus, for a certainnumber of particular values of d, an harmonic of an order greater than10 of the fundamental frequency is generated.

Finally, the tube is capable of producing a certain number of givenharmonic frequencies, of order greater than 10; therefore d is adjustedso as to obtain a given fundamental frequency, an harmonic of whichcorresponds to the desired output wavelength k After A has thus beenselected, the following adjustments must be performed:

The voltage of the reflector is first adjusted by trial and error, andcontingently, the load 34 connected to the output 30.

It is found that for a given dimensioning of the cavity resonator,several ranges of voltage of the reflector exist, each corresponding toa given adjustment of the load on the fundamental frequency. Inparticular, within the range corresponding to M, there exists a criticalvalue of voltage giving rise to a considerable harmonic power on M.Taking into account the desired high order of this harmonic, the valueto which the reflector voltage is finally adjusted is generally situatednear the threshold of the building up of fundamental oscillation.

Generally the impedance 34 is adjusted, for conven ience, in theneighborhood of a pure reactance for the oscillation t though this lastcondition is not, strictly speaking, necessary.

The distance h between the reflector and the upper edge of the opening13 is not critical. This distance is calculated, when designing thetube, and has a value equal to an integer number of A A being comprisedbetween A and A This distance h favors the generation of waves in theranges A and M. In the same manner, the trap 33 favors waves in theranges A and A if its depth is comprised between 4 and It may be assumedthat the reflected waves in the ranges x and are therefore totallyreflected by this trap and this reflector and return toward the cavityresonator, while the other waves continue to be propagated in thedirection of the cathode and are not reflected by the reflector. Thebeam is therefore modulated by the wave in the ranges of A and under.the most favorable conditions. The fins 14-14, 1616 favor the couplingbetween the beam and the waves in the ranges of A and M respectively.

The magnetic field H is also selected by trial and error for increasingthe output on the desired harmonics.

Finally in the klystron, according to the invention, the diameter of thebeam is of the order of one useful wavelength, whereas that of thecavity resonator is of the order of five times the useful wavelength.The latter is therefore readily obtained and, furthermore, the buildingup condition of the oscillations in the cavity resonator instead ofoccurring in respect of a millimeter wavelength occurs in respect of acentimeter wavelength; it is therefore easily satisfied.

The invention is not intended to be limited to the abovedescribedspecific embodiment.

What I claim is:

l. A reflex klystron oscillator for millimeter waves having an evacuatedand elongated envelope comprising a first cylindrical portion having afirst and a second end, a first plate perpendicular to its cylindricalaxis, closing said first end, extended at its center by a hollow conicalaxially positioned member and terminated by a first aperture and asecond aperture at its two respective ends; a second cylindrical portioncoaixal with said first cylindrical portion, having a first end and asecond end, a second plate perpendicular to its cylindrical axis closingits first end and having a third axially positioned aperture; a thirdportion constituting a cavity resonator bounded by said first plate,said hollow conical member and deformable walls; said cavity resonatorbeing dimensioned for resonating at a fundamental frequency and at leastat one predetermined higher frequency, in the millimeter range, andhaving a gap bounded by said second aperture and said third aperture;said ocillator further comprising: means for deforming said deformablewalls to tune said cavity resonator to a fundamental frequency which isa subharmonic of said predetermined higher frequency of an order atleast equal to ten; said first cylindrical portion including a source ofelectrons mounted adjacent said first aperture for projecting a beam ofelectrons through said hollow conical member, said gap and said thirdaperture; said gap being dimensioned for coupling said beam andultra-high frequency wave at said higher frequency; said secondcylindrical portion including a reflector electrode for reflectingelectrons back through said third aperture, said gap, and said conicalmember; at least a first output for energy at said fundamental frequencyand at least a second output for energy at said higher frequency.

2. A reflex klystron oscillator as claimed in claim 1, in which saidfirst output is loaded by a pure reactance.

3. A reflex klystron oscillator as claimed in claim 1, in which saidsecond output comprises at least one wave guide dimensioned to transmitsaid higher frequency.

4. A reflex klystron oscillator as claimed in claim '1, in which saidsecond and said third apertures are provided with at least one group ofradial fins, each of said groups having a radial dimension substantiallyequal to half the wavelength in free space of a particular higherresonance frequency of said cavity resonator and said second outputcomprising wave guides dimensions for said particular higherfrequencies.

5. A reflex klystron oscillator as claimed in claim 1, in which thedistance between said reflector electrode and said second aperture issubstantially equal to an integer number of half wavelengthscorresponding to said References Cited in the file of this patent UNITEDSTATES PATENTS 2,280,026 Brown Apr. 14, 1942 2,591,696 Hansen Apr. 8,1952 2,601,539 Marcum June 24, 1952

